1. Field of the Invention
The present invention relates to a method of performing a program operation of a phase change memory (PCM) cell. The present invention is further related to a corresponding pulse generator, a corresponding memory device and a corresponding computer program.
2. Description of the Related Art
Phase change memory (PCM) is a nonvolatile solid-state memory paradigm that exploits the property of reversible switching of certain chalcogenide materials between two stable states with very high resistivity contrast. The states of low/high resistivity are known as (poly-) crystalline and amorphous respectively. PCM has emerged in recent years as a potential successor of flash memory technology, because of its excellent read/write endurance, throughput performance, and future scaling. Multilevel functionality (multiple bits per unit memory cell) is a way to increase capacity without increasing cost, and thereby to reduce the cost per bit.
Storage of multiple resistance levels in a PCM cell is a challenge. Issues like process variability, as well as intra-cell and inter-cell material parameter variations give rise to deviations of the achieved resistance levels from their intended values. One way to resolve this issue is to resort to iterative programming schemes, with multiple write-verify steps until a desired resistance level is reached.
Bedeschi et al discloses in “A Bipolar-Selected Phase Change Memory Featuring Multi-Level Cell Storage”, IEEE Journal of Solid State Physics, VOL. 44, NO. 1, January 2009, a programming scheme that applies write pulses with heights that are incrementally increased to approach the target resistance.
US 2008/0084738 A1 discloses a method that starts in a crystalline mode and applies melting pulses to gradually amorphize.
The present invention provides another method of performing a program operation of a phase change memory cell.